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Ca II H and K Chromospheric Emission Lines in Late-K and M Dwarfs
We have measured the profiles of the Ca II H and K chromosphericemission lines in 147 main-sequence stars of spectral type M5-K7 (masses0.30-0.55 Msolar) using multiple high-resolution spectraobtained during 6 years with the HIRES spectrometer on the Keck Itelescope. Remarkably, the average FWHM, equivalent widths, and lineluminosities of Ca II H and K increase by a factor of 3 with increasingstellar mass over this small range of stellar masses. We fit the Ca II Hand K lines with a double-Gaussian model to represent both thechromospheric emission and the non-LTE central absorption. Most of thesample stars display a central absorption that is typically redshiftedby ~0.1 km s-1 relative to the emission. This implies thatthe higher level, lower density chromospheric material has a smalleroutward velocity (or higher inward velocity) by 0.1 km s-1than the lower level material in the chromosphere, but the nature ofthis velocity gradient remains unknown. The FWHM of the Ca II H and Kemission lines increase with stellar luminosity, reminiscent of theWilson-Bappu effect in FGK-type stars. Both the equivalent widths andFWHM exhibit modest temporal variability in individual stars. At a givenvalue of MV, stars exhibit a spread in both the equivalentwidth and FWHM of Ca II H and K, due both to a spread in fundamentalstellar parameters, including rotation rate, age, and possiblymetallicity, and to the spread in stellar mass at a given MV.The K line is consistently wider than the H line, as expected, and itscentral absorption is more redshifted, indicating that the H and K linesform at slightly different heights in the chromosphere where thevelocities are slightly different. The equivalent width of Hαcorrelates with Ca II H and K only for stars having Ca II equivalentwidths above ~2 Å, suggesting the existence of a magneticthreshold above which the lower and upper chromospheres become thermallycoupled.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. Keck time has been granted by both NASA and theUniversity of California.
| Exploring the Frequency of Close-in Jovian Planets around M Dwarfs
We discuss our high-precision radial velocity results of a sample of 90M dwarfs observed with the Hobby-Eberly Telescope and the Harlan J.Smith 2.7 m Telescope at McDonald Observatory, as well as the ESO VLTand the Keck I telescopes, within the context of the overall frequencyof Jupiter-mass planetary companions to main-sequence stars. None of thestars in our sample show variability indicative of a giant planet in ashort-period orbit, with a<=1 AU. We estimate an upper limit of thefrequency f of close-in Jovian planets around M dwarfs as <1.27% (atthe 1 σ confidence level). Furthermore, we determine that theefficiency of our survey in noticing planets in circular orbits is 98%for companions with msini>3.8MJ and a<=0.7 AU. Foreccentric orbits (e=0.6) the survey completeness is 95% for all planetswith msini>3.5MJ and a<=0.7 AU. Our results pointtoward a generally lower frequency of close-in Jovian planets for Mdwarfs as compared to FGK-type stars. This is an important piece ofinformation for our understanding of the process of planet formation asa function of stellar mass.Based on data collected with the Hobby-Eberly Telescope, which isoperated by McDonald Observatory on behalf of the University of Texas atAustin, Pennsylvania State University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen. Also based on observationscollected at the European Southern Observatory, Chile (ESO programs65.L-0428, 66.C-0446, 267.C-5700, 68.C-0415, 69.C-0722, 70.C-0044,71.C-0498, 072.C-0495, 173.C-0606). Additional data were obtained at theW. M. Keck Observatory, which is operated as a scientific partnershipamong the California Institute of Technology, the University ofCalifornia, and the National Aeronautics and Space Administration(NASA), and with the McDonald Observatory Harlan J. Smith 2.7 mtelescope.
| The structure of our stellar system.
Not Available
| A New Detached M Dwarf Eclipsing Binary
We describe a newly discovered detached M dwarf eclipsing binary system.This system was first observed by the TrES network during a long-termphotometry campaign of 54 nights. Analysis of the folded light curveindicates two very similar components orbiting each other with a periodof 1.12079 +/- 0.00001 days. Spectroscopic observations with theHobby-Eberly Telescope show the system to consist of two M3e dwarfs in anear-circular orbit. Double-line radial velocity amplitudes, combinedwith the orbital inclination derived from light-curve fitting, yieldMtotal = 0.983 +/- 0.007 Msolar, with componentmasses of M1=0.493+/-0.003 Msolar andM2=0.489+/-0.003 Msolar. The light-curve fityields component radii of R1=0.453+/-0.060 Rsolarand R2=0.452+/-0.050 Rsolar. Although a preciseparallax is lacking, broadband VJHK colors and spectral typing suggestcomponent absolute magnitudes of MV(1)=11.18+/-0.30 andMV(2)=11.28+/-0.30.
| Astrometric orbits of SB^9 stars
Hipparcos Intermediate Astrometric Data (IAD) have been used to deriveastrometric orbital elements for spectroscopic binaries from the newlyreleased Ninth Catalogue of Spectroscopic Binary Orbits(SB^9). This endeavour is justified by the fact that (i) theastrometric orbital motion is often difficult to detect without theprior knowledge of the spectroscopic orbital elements, and (ii) suchknowledge was not available at the time of the construction of theHipparcos Catalogue for the spectroscopic binaries which were recentlyadded to the SB^9 catalogue. Among the 1374 binaries fromSB^9 which have an HIP entry (excluding binaries with visualcompanions, or DMSA/C in the Double and Multiple Stars Annex), 282 havedetectable orbital astrometric motion (at the 5% significance level).Among those, only 70 have astrometric orbital elements that are reliablydetermined (according to specific statistical tests), and for the firsttime for 20 systems. This represents a 8.5% increase of the number ofastrometric systems with known orbital elements (The Double and MultipleSystems Annex contains 235 of those DMSA/O systems). The detection ofthe astrometric orbital motion when the Hipparcos IAD are supplementedby the spectroscopic orbital elements is close to 100% for binaries withonly one visible component, provided that the period is in the 50-1000 drange and the parallax is >5 mas. This result is an interestingtestbed to guide the choice of algorithms and statistical tests to beused in the search for astrometric binaries during the forthcoming ESAGaia mission. Finally, orbital inclinations provided by the presentanalysis have been used to derive several astrophysical quantities. Forinstance, 29 among the 70 systems with reliable astrometric orbitalelements involve main sequence stars for which the companion mass couldbe derived. Some interesting conclusions may be drawn from this new setof stellar masses, like the enigmatic nature of the companion to theHyades F dwarf HIP 20935. This system has a mass ratio of 0.98 but thecompanion remains elusive.
| Astrometric Discovery of GJ 164B
We discovered a low-mass companion to the M dwarf GJ 164 with theCCD-based imaging system of the Stellar Planet Survey astrometricprogram. The existence of GJ 164B was confirmed with Hubble SpaceTelescope NICMOS imaging observations. A high-dispersion spectralobservation in V sets a lower limit of Δm>2.2 mag between thetwo components of the system. Based on our parallax value of 82+/-8 mas,we derive the following orbital parameters: P=2.04+/-0.03 yr,a=1.03+/-0.03, and Mtotal=0.265+/-0.020 Msolar.The component masses are MA=0.170+/-0.015 Msolarand MB=0.095+/-0.015 Msolar. Based on its mass,colors, and spectral properties, GJ 164B has spectral type M6-M8 V.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| A Dedicated M Dwarf Planet Search Using The Hobby-Eberly Telescope
We present the first results from our planet-search program using the9.2 m Hobby-Eberly Telescope (HET) at McDonald Observatory to detectplanets around M-type dwarf stars by means of high-precision radialvelocity (RV) measurements. Although more than 100 extrasolar planetshave been found around solar-type stars of spectral type F-K, there isonly a single M dwarf (GJ 876) known to harbor a planetary system. Withthe current incompleteness of Doppler surveys with respect to M dwarfs,it is not yet possible to decide whether this is due to a fundamentaldifference in the formation history and overall frequency of planetarysystems in the low-mass regime of the Hertzsprung-Russell diagram, orsimply an observational bias. Our HET M dwarf survey plans to survey 100M dwarfs in the next 3 to 4 years, with the primary goal being to answerthis question. Here we present the results from the first year of thesurvey, which show that our routine RV precision for M dwarfs is 6 ms-1. We found that GJ 864 and GJ 913 are binary systems withas yet undetermined periods, while five out of 39 M dwarfs reveal a highRV scatter and represent candidates for having short-period planetarycompanions. For one of them, GJ 436 (rms=20.6 m s-1), we havealready obtained follow-up observations, but no periodic signal ispresent in the RV data.Based on data collected with the Hobby-Eberly Telescope, which isoperated by McDonald Observatory on behalf of the University of Texas atAustin, Pennsylvania State University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| The radii and spectra of the nearest stars
We discuss direct measurements of the radii of 36 stars located closerthan 25 parsecs to the Sun. We present the data on 307 radii and 326spectral types and luminosity classes for the nearest stars locatedinside the sphere with a radius of 10 parsecs.
| Radial Velocities for 889 Late-Type Stars
We report radial velocities for 844 FGKM-type main-sequence and subgiantstars and 45 K giants, most of which had either low-precision velocitymeasurements or none at all. These velocities differ from the standardstars of Udry et al. by 0.035 km s-1 (rms) for the 26 FGKstandard stars in common. The zero point of our velocities differs fromthat of Udry et al.: =+0.053km s-1. Thus, these new velocities agree with the best knownstandard stars both in precision and zero point, to well within 0.1 kms-1. Nonetheless, both these velocities and the standardssuffer from three sources of systematic error, namely, convectiveblueshift, gravitational redshift, and spectral type mismatch of thereference spectrum. These systematic errors are here forced to be zerofor G2 V stars by using the Sun as reference, with Vesta and day sky asproxies. But for spectral types departing from solar, the systematicerrors reach 0.3 km s-1 in the F and K stars and 0.4 kms-1 in M dwarfs. Multiple spectra were obtained for all 889stars during 4 years, and 782 of them exhibit velocity scatter less than0.1 km s-1. These stars may serve as radial velocitystandards if they remain constant in velocity. We found 11 newspectroscopic binaries and report orbital parameters for them. Based onobservations obtained at the W. M. Keck Observatory, which is operatedjointly by the University of California and the California Institute ofTechnology, and on observations obtained at the Lick Observatory, whichis operated by the University of California.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| The Palomar/MSU Nearby Star Spectroscopic Survey. III. Chromospheric Activity, M Dwarf Ages, and the Local Star Formation History
We present high-resolution echelle spectroscopy of 676 nearby M dwarfs.Our measurements include radial velocities, equivalent widths ofimportant chromospheric emission lines, and rotational velocities forrapidly rotating stars. We identify several distinct groups by theirHα properties and investigate variations in chromospheric activityamong early (M0-M2.5) and mid (M3-M6) dwarfs. Using a volume-limitedsample together with a relationship between age and chromosphericactivity, we show that the rate of star formation in the immediate solarneighborhood has been relatively constant over the last 4 Gyr. Inparticular, our results are inconsistent with recent large bursts ofstar formation. We use the correlation between Hα activity and ageas a function of color to set constraints on the properties of L and Tdwarf secondary components in binary systems. We also identify a numberof interesting stars, including rapid rotators, radial velocityvariables, and spectroscopic binaries. Observations were made at the 60inch telescope at Palomar Mountain, which is jointly owned by theCalifornia Institute of Technology and the Carnegie Institution ofWashington.
| Meeting the Cool Neighbors. I. Nearby Stars in the NLTT Catalogue: Defining the Sample
We are currently undertaking a program aimed at identifying previouslyunrecognized late-type dwarfs within 20 pc of the Sun. As a first step,we have cross-referenced Luyten's NLTT proper-motion catalog against thesecond incremental release of the Two Micron All Sky Survey (2MASS)Point Source Catalog and use optical/infrared colors, derived bycombining Luyten's mr estimates with 2MASS data, to identifycandidate nearby stars. This paper describes the definition of areference sample of 1245 stars and presents a compilation of literaturedata for more than one-third of the sample. Only 274 stars havetrigonometric parallax measurements, but we have used data for nearbystars with well-determined trigonometric parallaxes to computecolor-magnitude relations in the (MV, V-K), (MV,V-I), and (MI, I-J) planes and use those relations todetermine photometric parallaxes for NLTT stars with optical photometry.Based on the 2MASS JHKs data alone, we have identified afurther 42 ultracool dwarfs (J-Ks>0.99) and useJ-Ks colors to estimate photometric parallaxes. Combiningthese various techniques, we identify 308 stars with formal distances ofless than 20 pc, while a further 46 have distance estimates within 1σ of our survey limit. Of these 354 stars, 75, including 39 of theultracool dwarfs, are new to nearby-star catalogs. Two stars with bothoptical and near-infrared photometry are potential additions to theimmediate solar neighborhood, with formal distance estimates of lessthan 10 pc.
| A Near-Infrared, Wide-Field, Proper-Motion Search for Brown Dwarfs
A common proper-motion survey of M dwarf stars within 8 pc of the Sunreveals no new stellar or brown dwarf companions at wide separations(~100-1400 AU). This survey tests whether the brown dwarf ``desert''extends to large separations around M dwarf stars and further exploresthe census of the solar neighborhood. The sample includes 66 stars northof -30° and within 8 pc of the Sun. Existing first-epoch images arecompared with new J-band images of the same fields an average of 7 yrlater to reveal proper-motion companions within a ~4' radius of theprimary star. No new companions are detected to a J-band limitingmagnitude of ~16.5, corresponding to a companion mass of ~40 Jupitermasses for an assumed age of 5 Gyr at the mean distance of the objectsin the survey, 5.8 pc.
| The Solar Neighborhood. VI. New Southern Nearby Stars Identified by Optical Spectroscopy
Broadband optical spectra are presented for 34 known and candidatenearby stars in the southern sky. Spectral types are determined using anew method that compares the entire spectrum with spectra of more than100 standard stars. We estimate distances to 13 candidate nearby starsusing our spectra and new or published photometry. Six of these starsare probably within 25 pc, and two are likely to be within the ResearchConsortium on Nearby Stars (RECONS) horizon of 10 pc.
| Accurate masses of very low mass stars. IV. Improved mass-luminosity relations
We present improved visual and near-infrared empirical mass-luminosityrelations for very low mass stars (M<0.6 Msolar). Theserelations make use of all stellar masses in this range known with betterthan 10% accuracy, most of which are new determinations with 0.2 to 5%accuracy from our own programme, presented in a companion paper. Aspredicted by stellar structure models, the metallicity dispersion of thefield populations induces a large scatter around the mean V bandrelation, while the infrared relations are much tighter. The agreementof the observed infrared mass-luminosity relations with the theoreticalrelations of Baraffe et al. (\cite{baraffe98}) and Siess et al.(\cite{siess00}) is impressive, while we find an increasinglysignificant discrepancy in the V band for decreasing masses. Thetheoretical mass-luminosity relation which is insufficiently steep, andhas introduced significant errors in the local stellar mass functionsderived from V band luminosity functions. Based on observations made atthe Observatoire de Haute Provence (CNRS), and at the CFH Telescope,operated by the NRCC, the CNRS and the University of Hawaii
| Exploring the brown dwarf desert with Hipparcos
The orbital elements of 11 spectroscopic binaries with brown dwarfcandidates (M2 sin i between 0.01 and 0.08 Msun)are combined with the Hipparcos observations in order to deriveastrometric orbits. Estimations of the masses of the secondarycomponents are thus calculated. It appears that 5 secondary masses aremore than 2 sigmaM2 above the limit of 0.08Msun, and are therefore not brown dwarfs. 2 other stars arestill discarded at the 1 sigmaM2 level, 1 browndwarf is accepted with a low confidence, and we are finally left with 3viable candidates which must be studied by other means. A statisticalapproach is developed, based on the relation between the semi-major axesof the photocentric orbit, a_0, their errors, sigma a_0, andthe frequency distribution of the mass ratios, q. It is investigatedwhether the set of values of a_0 and sigma a_0 obtained forthe sample is compatible with different frequency distributions of q. Itis concluded that a minimum actually exists for M2 betweenabout 0.01 and 0.1 Msun for companions of solar-type stars.This feature could correspond to the transition between giant planetsand stellar companions. Due to the relatively large frequency of singlebrown dwarfs found recently in open clusters, it is concluded that thedistribution of the masses of the secondary components in binary systemsdoes not correspond to the IMF, at least for masses below thehydrogen-ignition limit. Based on photoelectric radial-velocitymeasurements collected at Haute-Provence observatory and on observationsmade with the ESA Hipparcos astrometry satellite.
| L Dwarfs and the Substellar Mass Function
Analysis of initial observations sky surveys has shown that theresulting photometric catalogs, combined with far-red optical data,provide an extremely effective method of finding isolated, verylow-temperature objects in the general field. Follow-up observationshave already identified more than 25 sources with temperatures coolerthan the latest M dwarfs. A comparison with detailed model predictions(Burrows & Sharp 1999) indicates that these L dwarfs have effectivetemperatures between ~2000+/-100 K and 1500+/-100 K, while the availabletrigonometric parallax data place their luminosities at between 10^-3.5and 10. Those properties, together with the detection of lithium inone-third of the objects, are consistent with the majority havingsubstellar masses. The mass function cannot be derived directly, sinceonly near-infrared photometry and spectral types are available for mostsources, but we can incorporate VLM/brown dwarf models in simulations ofthe solar neighborhood population and constrain Psi(M) by comparing thepredicted L dwarf surface densities and temperature distributionsagainst observations from the Deep Near-Infrared Survey (DENIS) and 2Micron All-Sky Survey (2MASS) surveys. The data, although sparse, can berepresented by a power-law mass function, Psi(M)~M^-alpha, with1M/M_solar>0.01brown dwarfs is 0.10 systems pc^-3. In that case, brown dwarfs are twiceas common as main-sequence stars but contribute no more than ~15% of thetotal mass of the disk.
| The Optical Mass-Luminosity Relation at the End of the Main Sequence (0.08-0.20 M_solar)
The empirical mass-luminosity relation at M_V is presented for starswith masses 0.08-0.20 M_solar based upon new observations made with FineGuidance Sensor 3 on the Hubble Space Telescope. The targets are nearby,red dwarf multiple systems in which the magnitude differences aretypically measured to +/-0.1 mag or better. The M_V values are generatedusing the best available parallaxes and are also accurate to +/-0.1 mag,because the errors in the magnitude differences are the dominant errorsource. In several cases this is the first time the observedsub-arcsecond multiples have been resolved at optical wavelengths. Themass-luminosity relation defined by these data reaches to M_V=18.5 andprovides a powerful empirical test for discriminating the lowest massstars from high-mass brown dwarfs at wavelengths shorter than 1 mum.
| Photometric Measurements of the Fields of More than 700 Nearby Stars
In preparation for optical/IR interferometric searches for substellarcompanions of nearby stars, we undertook to characterize the fields ofall nearby stars visible from the Northern Hemisphere to determinesuitable companions for interferometric phase referencing. Because theKeck Interferometer in particular will be able to phase-reference oncompanions within the isoplanatic patch (30") to about 17th magnitude atK, we took images at V, r, and i that were deep enough to determine iffield stars were present to this magnitude around nearby stars using aspot-coated CCD. We report on 733 fields containing 10,629 measurementsin up to three filters (Gunn i, r and Johnson V) of nearby stars down toabout 13th magnitude at V.
| Photometry of Proxima Centauri and Barnard's Star Using Hubble Space Telescope Fine Guidance Sensor 3: A Search for Periodic Variations
We have observed Proxima Centauri and Barnard's star with the HubbleSpace Telescope Fine Guidance Sensor 3. Proxima Cen exhibitssmall-amplitude, periodic photometric variations. Once several sourcesof systematic photometric error are corrected, we obtain 2 mmag internalphotometric precision. We identify two distinct behavior modes over thepast 4 years: higher amplitude, longer period and smaller amplitude,shorter period. Within the errors, one period (P ~ 83 days) is twice theother. Barnard's star shows very weak evidence for periodicity on atimescale of approximately 130 days. If we interpret these periodicphenomena as rotational modulation of starspots, we identify threediscrete spots on Proxima Cen and possibly one spot on Barnard's star.We find that the disturbances change significantly on timescales asshort as one rotation period. Based on observations with the NASA/ESAHubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Library of high-resolution UES echelle spectra of F, G, K and M field dwarf stars
We present a library of Utrecht echelle spectrograph (UES) observationsof a sample of F, G, K and M field dwarf stars covering the spectralrange from 4800 \ Angstroms to 10600 \ Angstroms with a resolution of55000. These spectra include some of the spectral lines most widely usedas optical and near-infrared indicators of chromospheric activity suchas Hβ , Mg i b triplet, Na i D1, D2, He iD3, Hα , and Ca ii IRT lines, as well as a large numberof photospheric lines which can also be affected by chromosphericactivity. The spectra have been compiled with the aim of providing a setof standards observed at high-resolution to be used in the applicationof the spectral subtraction technique to obtain the active-chromospherecontribution to these lines in chromospherically active single andbinary stars. This library can also be used for spectral classificationpurposes. A digital version with all the spectra is available via ftpand the World Wide Web (WWW) in both ASCII and FITS formats. Based onobservations made with the William Herschel Telescope operated on theisland of La Palma by the Isaac Newton Group at the Spanish Observatoriodel Roque de Los Muchachos of the Instituto de Astrof\'{\i
| Rotation and chromospheric activity in field M dwarfs
We have obtained high resolution spectra for a volume-limited sample of118 field M dwarfs. From these observations we derive projectedrotational velocities and fluxes in the H_alpha and H_beta lines. 8stars are double-lined spectroscopic binaries with measured or probableperiods short enough for rotation to be tidally synchronized with theorbit, and another 11 are visual binaries where we cannot yet separatethe lines of the two stars. Of the remaining 99 stars, 24 haverotational velocities above our detection limit of ~ 2 km.s(-1) , andsome are quite fast rotators, including two with v sin i\ =~ 30 km.s(-1)and one with v sin i\ =~ 50 km.s(-1) . Given the small radii of Mdwarfs, these moderate rotational velocities correspond to rather shortmaximum rotational periods, of only 7-8 hours. These three stars aregood candidates for Doppler imaging. We find that rotation is stronglycorrelated with both spectral type and kinematic population: all starswith measurable rotation are later than M3.5, and all but one havekinematic properties typical of the young disk, or intermediate betweenthe young disk and the the old disk. We interpret this correlation asevidence for a spin-down timescale that increases with decreasing mass.At the age of the old disk or halo, all stars earlier than M5-M6(0.1-0.15Msun) have spun-down to below our detection limit,while at the age of the young disk this has only happened for starsearlier than M3.5. The one star with measurable rotation and akinematics intermediate between old disk and population II has spectraltype M6. It is probably an old star whose mass is low enough that it hasretained significant rotation up to present, still consistently withlonger spin-down times for lower mass stars. We observe, on the otherhand, no conspicuous change in the v sin i\ distribution or activitypattern at the mass (M ~ 0.35 Msun) below which stars remainfully convective down to the main sequence. These new data areconsistent with a saturated correlation between rotation and activity,similar to the one observed for younger or more massive stars:L_X/Lbol and L_{H_alpha }/Lbol both correlate withv sin i\ for v sin i\ -5km.s^{-1} and then saturate at respectively10^{-2.5} and 10^{-3.5}$. Based on observations made at the Observatoirede Haute-Provence (CNRS), France Tables 2 and 4 are also available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html.
| Mass determination of astrometric binaries with Hipparcos. II. Selection of candidates and results
In a previous paper (\cite{Mar97}) we have shown that for double starswith orbital periods smaller than about 25 years, it was possible todetermine from the Hipparcos data, the mass ratio B of the components orthe difference between the mass and intensity ratios, beta -B, providedthe orbital elements of the relative orbit are available. From anextensive literature search we have selected 145 potential systems, ofwhich 46 yielded eventually a satisfactory solution. For eight systemswith the largest separations, the peculiarities of the natural directionassociated to the Hipparcos observations, the 'hippacentre', have beenfully exploited to derive the mass ratio of the components without anyadditional assumption. For the remaining 38, the derivation of the massratio was possible only by taking the magnitude difference between thetwo components from other sources. The parallax determinedsimultaneously, is then used to produce the individual masses of thecomponents. The astrophysical relevance of the results is discussed andwhen possible (17 systems) the masses are compared to ground-basedvalues.
| Mass determination of astrometric binaries with Hipparcos. I. Theory and simulation
The analysis of the observations of double stars made by the ESAsatellite Hipparcos has involved a very specific processing to derivethe relevant astrometric parameters. This required to distinguishbetween several categories of double stars according to the separationand orbital motion. We show that for close pairs with orbital periodless than about 20 years, the concept of photocentric orbit of anastrometric binary needs to be generalized to benefit fully from theaccuracy of Hipparcos. We introduce a point more naturally associatedwith the Hipparcos observations, the hippacentre, whose orbital path isnot longer similar to the relative keplerian orbit of the components,unlike that of the photocentre. For systems with separation larger thanabout 0.3", it is possible to determine separately the mass and theintensity ratio of the components from the absolute path of thehippacentre on the sky. For smaller separations the scale of thephotocentric orbit is recovered as a limiting case. The scope of thispaper is to set forth the principles of the method and to explore itspossibilities and limitations from extensive simulations. Based onobservations made by the ESA Hipparcos satellite.
| Mass-luminosity relation of low mass stars.
The data on dynamic masses and multicolor photometry of 56 M-typecomponents of binary/multiple systems was collected. Critical evaluationof late type stars bolometric correction scales have been performed. Ourrefined and reduced data is compared with published empirical andtheoretical mass-luminosity relations. Our data does not exclude theexistence of a step-like feature at M_V_=12mag. The best agreementbetween observations and theoretical models is found for recentcalculations of D'Antona & Mazzitelli (1994ApJS...90..467D) withAlexander opacities. We conclude that present-day knowledge of themass-luminosity relation at faintest magnitudes is not sufficient formaking definite conclusions on the initial mass function of low massstars.
| The Palomar/MSU Nearby Star Spectroscopic Survey.II.The Southern M Dwarfs and Investigation of Magnetic Activity
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.2799H&db_key=AST
| Photometry of Stars with Large Proper Motion
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.2300W&db_key=AST
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| Costellazione: | Ercole |
| Ascensione retta: | 16h24m09.32s |
| Declinazione: | +48°21'10.5" |
| Magnitudine apparente: | 10.277 |
| Distanza: | 8.042 parsec |
| Moto proprio RA: | 1144.2 |
| Moto proprio Dec: | -450.8 |
| B-T magnitude: | 12.232 |
| V-T magnitude: | 10.439 |
Cataloghi e designazioni:
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